Segmentation fault when free the variable after using strt_ok

2
votes

I have segmentation fault when executing this simple program (it is just a light version to reproduce the error).

//   gcc main.c -Wall -Wextra -Wpedantic
//   ./a.out

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h> // uint32_t

int main(){

    char* rest = (char*)malloc(128 * sizeof(char));
    char* token= (char*)malloc(128 * sizeof(char)); 
    strcpy(rest,"Something_else");
    token = strtok_r(rest, "_", &rest);
    printf("%s\n", token);
    free(token);
    free(rest);
    return 0;
}

The free of the variable token does not give any error. The free of the variable rest gives me always a segmentation fault every time I use the function strok_r. What is going on? Any suggestion? No warnings are prompt at compilation time.

Question:

How to re-write this simple code properly?

2
csegmentation-faultmallocfree
You are using rest both as save pointer and as string to tokenise. That means that rest will now point to "else", but your free needs the exact pointer that was allocated, namely the one to "Something_else". - M Oehm

2 Answers

5
votes

You only need memory for the sentence, token and rest are just pointers.

And using a while loop, you can see all the tokens:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h> // uint32_t

int main(){

    char* rest , *token;
    char* setence= malloc(128 * sizeof(char)); 

    strcpy(setence, "Some_thing_else");

    token = strtok_r(setence, "_", &rest);

    while (token) 
    {
        printf("%s\n", token);
        token = strtok_r(NULL, "_", &rest);          
    }

    free(setence);

    return 0;
}

Will give:

Some
thing
else
2
votes

You should neither allocate nor free the variable 'rest'. strtok_r uses its third argument to store where it got to in the original string. So:

char* rest;
char* copy= (char*)malloc(128 * sizeof(char)); 
strcpy(copy,"Something_else");
char* token = strtok_r(copy, "_", &rest); 
// ...
free( copy);  // don't free token or rest